CONDITIONS OF RHODOCHROSITE-NODULE FORMATION IN NEOGENE-PLEISTOCENE DEEP-SEA SEDIMENTS - EVIDENCE FROM O, C AND SR ISOTOPES

Calcic rhodochrosite occurs as nodules around burrows in late Neogene- early Pleistocene pelagic sediments of the Galapagos Ridge in the Guat emala Basin, eastern equatorial Pacific (DSDP Leg 68; Site 503). Growt h was accomplished in the suboxic zone (delta(13)C = -4.3 parts per th ousand to -1.3 parts per thousand) at an estimated depth between appro ximate to 30 and 70 cm beneath the seafloor. The rhodochrosites have S r-87/Sr-86 ratios (0.708945-0.709073) that largely record the small-sc ale changes in seawater Sr-isotopic composition. The anomalous enrichm ent of the rhodochrosite in O-18 isotopes (delta(18)O(PDB) UP to +6.4 parts per thousand) is attributed to the dissociation of gas hydrates.